1. Field of the Invention
This invention relates to the field of apparatus for controlling the focus of a light beam in an optical data storage system.
2. Background Art
Optical disk recording and reproducing systems store information, such as video, voice, digital and other data, on a disk in a series of concentric or spiral "tracks". The disk is rotated at high speed and a light, such as a laser or any other suitable, detectable radiation source, is utilized to scan the disk. The light is reflected from the data tracks and modulated by the data information stored in the tracks. This modulated and reflected light is detected and decoded to reproduce the information stored on the disk.
Recently, optical disks have become a viable means for storage of electronic information and for use in memory systems for computer related activities. Two forms of such optical memories are known as optical ROM's (read only memories) or WORM's (write once, read many times). In addition, development is under way of optical disks that have the functional capability of magnetic hard disk or floppy disk storage systems; namely, the information on these new optical disks may be erased, written and/or read many times.
Typically, the cost per unit capacity is critical in order to keep the cost of optical disk storage systems low. It is desired to store information at high area density on the optical disk. In optical disk systems, information density is dependant on the area of the focused light spot incident on the disk, which light is utilized in reading information from the disk. To achieve a small area focused spot in high density optical disk systems, an objective lens system with a large numerical aperture is used, having an associated depth of focus that is small, often on the order of microns.
In modern optical disk media, invariably there is displacement of the disk surface at any given point on the disk during rotation in both the axial and radial directions resulting from disk processing variations and disk mounting error. As a result the focus spot position of the light beam must be repeatedly corrected using a focus servo system in the axial direction and a tracking servo system in the radial direction.
Although the light reflected from the disk to be irradiated is utilized for the focal point detection light beam, the transmitted light may be utilized for the same purpose from a similar technical point of view.